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The symbols log2, lb or ld are used for the binary logarithm, also called dual logarithm.

Log2 of the first primes

prime
log2 prime
2
1
3
1.584962501
5
2.321928095
7
2.807354922
11
3.459431619
13
3.700439718
17
4.087462841
19
4.247927513
23
4.523561956
29
4.857980995

You can calculate the binary logarithm of n like this

log2(n) = ln(n)/ln(2)